When an atom of a fissionable substance, especially uranium-235, absorbs a neutron in its core and undergoes disintegration, there are formed, on average, two fission fragments with lower atomic weight and greater kinetic energy as well as a plurality of high-energetic neutrons. In a reactor core a sufficient number of assemblies with nuclear fuel are arranged to make possible a self-supporting fission reaction. The kinetic energy of the fission products escape as heat from the fuel rods. The reactor core is immersed in a coolant, for example water, which discharges the heat for utilization thereof. When the coolant is in the form of water, it also serves as a neutron moderator which retards the neutrons so as to increase the probability of fission reactions. If the reactor is to operate at a constant power level, the amount of fission-generating neutrons must be constant. This means that each fission reaction must generate one neutron net, which in turn gives rise to a subsequent fission reaction such that the process becomes self-supporting. This is usually expressed such that the effective multiplication factor k.sub.eff must be equal to 1. The multiplication factor describes the ratio of the number of produced neutrons to the number of absorbed neutrons (or neutrons leaking out of the system).
During operation, the fissionable material is depleted while at the same time some of the fission products themselves are neutron-absorbing. Considering this fact, the reactor is normally provided from the start with an operating cycle with an excess of nuclear fuel, which initially entails too high a reactivity. For this reason, a control system is required which is capable both of maintaining the effective multiplication factor k.sub.eff exactly at 1.0 during operation and of reducing it to below 1 when the reactor is to be shut down. An important part of this reactivity control is taken care of by neutron-absorbing material, which absorbs or captures neutrons without any fission taking place.
At least part of the neutron-absorbing material is included in a plurality of the selectively operable control rods, which are pushed up from the bottom of the core to the necessary extent for control of the power level thereof and of the power distribution as well as for shutdown of the reactor. When the control rods are inserted into the core, the neutrons are absorbed which are a condition for the nuclear fission, the reactivity thus dropping. The higher neutron-absorbing effect the control rod has, the better is the so-called control rod effect thereof.
Some of the fuel rods may contain burnable absorption material for reducing the need of mechanical control. Such burnable absorber is transformed by absorption of neutrons into a material with a lower neutron-absorbing capacity. A well-known such material is gadolinium, usually in the form of gadolinium oxide. The burnable absorbers which are available as construction material, however, have a non-negligible residual absorption capacity. When using, for example, gadolinium as burnable absorber, the isotopes which have a high neutron capture cross section will be consumed relatively fast, whereas a residual absorption capacity remains as a result of continued neutron capture of the other isotopes.
When the need arises, the power production in the core must be capable of being rapidly interrupted, that is, the neutron supply and hence the power generation from nuclear fissions in the fuel be interrupted. There must always be sufficient shutdown margins such that the neutron supply does not unexpectedly start, resulting in powerful power generation, for example when the reactor vessel is opened and service work or refuelling is in progress.
A typical requirement by the authorities for operational approval is that if any one of the control rods has stuck in its outer position, then the shutdown margin shall correspond to a reactivity reduction of at least 0.38% (k.sub.eff is to be less than 0.9962). To obtain additional safety, these values are in practice often changed to 1% and 0.99, respectively.
It is known to improve the shutdown margin by incorporating some burnable neutron absorber in the fuel pellets, for example gadolinium. The burnable neutron absorber provides a reduction of the reactivity in both a cold and a hot state. Incorporating burnable absorbers in the fuel pellets is costly and, in addition, the burnable absorbers cannot be burnt up completely, which means that a certain percentage of neutron-absorbing material always remains, which reduces the reactivity in the hot operating state, which is not desirable.
An additional problem is that burnable neutron absorbers such as gadolinium oxide reduce the thermal conductivity of the fuel rods. Fuel rods which contain gadolinium oxide will have a considerably lower relative power because of the absorber, which has an unfavourable influence on the local power distribution. The larger the number of rods with burnable absorber and the larger the concentrations of burnable absorber, the greater will be the negative effect on the local power distribution.
To sum up, thus, the requirements imposed on the reactor core during operation and during shutdown often act in opposite directions, which has made the design of a core with an optimum configuration difficult.
Some of the known configurations, in which an improved shutdown has been aimed at, will be described below.
U.S. Pat. No. 4,863,680 discloses a fuel assembly in which an increased shutdown margin is achieved by arranging in the fuel assembly a number of small units with a small number of fuel rods in each unit. The units are arranged in a specific spaced relationship to each other. Centrally among the small units, a water rod is arranged. The shutdown margin can be ensured by varying the distances between the units in a suitable way.
U.S. Pat. No. 4,968,479 discloses a fuel assembly with a number of partial-length rods arranged around a centrally located water rod. The water rod has an upper part with a larger diameter and a lower part with a smaller diameter, where the smaller diameter substantially corresponds to the diameter of the fuel rods. Some of the rods are provided with intermediate zones of non-fissile material. These zones are arranged around the upper part of the water rod such that the effective multiplication factor, k.sub.eff, in hot state can be effectively increased and in cold state be effectively reduced, whereby an improved shutdown margin is obtained. This is due to the fact that there is an excess of water around the water rod at the intermediate zones such that the water rod or the region around this rod is overmoderated in cold state, the neutron multiplication factor thus decreasing and the shutdown margin increasing. During the hot state of the reactor, especially when steam bubbles appear at the outer periphery of the water rod, the excessive water will disappear and the multiplication factor will recover.
U.S. Pat. No. 5,128,097 shows a fuel assembly which comprises central fuel rods arranged in a square lattice with a larger diameter than peripheral fuel rods arranged in an triangular lattice. The peripheral triangular lattice pattern makes it possible to increase the cooling region at the periphery, whereby the shutdown margin is increased. The amount of coolant at the centre of the fuel assembly is increased by the introduction of two water rods with an enlarged diameter in relation to the fuel rods.
SE 454 822 discloses a fuel assembly which comprises four sub-assemblies each provided with a reduced corner portion, wherein the reduced corner portions are facing each other forming an enlarged centre in the fuel assembly. The sub-assemblies are separated by a cruciform support means, the cruciform centre of which has been enlarged when being adapted to the reduced corner portions. The enlarged centre of the channel-formed support means contributes to the possibility of containing more non-boiling water in the centre part of the fuel assembly. The shutdown margin is thus improved in a cold reactor by containing the larger amount of water in the central part of the fuel assembly.
SE 423 760 discloses another fuel assembly with reduced corner portions. The fuel assembly comprises four subassemblies, of which at least one is provided with four reduced corner portions. The reason for the corner reduction in this design is a desire to accommodate a further fuel rod in a limited space. To achieve this, the rods are arranged in a partially triangular pattern instead of in a square pattern. It is then natural to adapt the corner portions of the sub-assembly to the triangular rod configuration. This fuel assembly gives no improved shutdown margin. Admittedly, more water is let into the core but it provides no reduction of the reactivity in cold state since the ratio of water to uranium is not changed.